The invention relates to a soldering nest for a bus bar, and may find primary use in power-current devices including semiconductors and in power-current control systems.
In power-current systems which utilize copper or brass bus bars to conduct currents of at least 10 A, semiconductor devices and the ends of connecting wires are joined to the bus bar by soldering. Based on the examination of defect statistics and manufacturing waste, it has been found that most of the defects occur at the locations of soldering. The defects may be divided into two main groups: defects of the semiconductor elements occurring during soldering or as a result thereof and defects which occur at the location of soldering because of an imperfect contact and which are noticeable only after a prolonged service.
In preparation of soldering, for the cylindrical elements to be soldered (wire ends or electrode terminal leads of semiconductors), in the flat bus bars cylindrical bores of a diameter greater than the wire diameter of the elements are provided. Occasionally, the bores are treated by galvanization. Thereafter the wire ends are placed into the bores before soldering. Heat is transmitted, usually with the interposition of molten soldering tin, by a soldering iron or other suitable soldering tool placed on that side of the bus bar which is opposite the location where the elements were introduced.
Problems are encountered because the bus bar is a good heat conductor and a heat transmission for an extended period is needed to obtain the required temperature through the bore or gap for suitably melting the soldering material. In case heat transmission occurs for a short period of time, no bond is obtained along a suitably large surface, and while some electric connection will be present, its transitional resistance will be large. This circumstance gives rise to defects—which manifest themselves only in the course of prolonged use—because the larger transitional resistance resulting from the smaller connecting area causes a local heat-up which further damages the contact, rendering the defect noticeable sooner or later. If the soldering iron is maintained in place for a longer period than necessary to obtain the suitable temperature, the terminal lead of the semiconductor is exposed for a long period to the effect of the greater heat and, as a consequence of the good heat conductivity, it is the semiconductor which will be adversely affected because of the prolonged high temperature. An optimal situation is highly uncertain and depends from many factors that are difficult to control.
It is an object of the invention to provide a soldering nest which results in a substantially more reliable and more easily obtainable bond between the bus bar and the cylindrical element to be soldered and where the heat affecting the semiconductor is more moderate.